TERRESTRIAL PALEOCLIMATOLOGY OF THE MID-CRETACEOUS GREENHOUSE II: OXYGEN ISOTOPIC EVIDENCE FOR ENHANCED ATMOSPHERIC HEAT TRANSPORT

Terrestrial "greenhouse-world" paleosol-carbonate proxy records and quantitative reconstructions of the mid-Cretaceous hydrologic cycle show that heat transfer through the atmosphere via water vapor played a greater role in cooling the tropics and warming the high latitudes than at present, and represents a viable alternative to oceanic heat transport. Precipitation-evaporation (P-E) balances define latitudinal zones with moisture deficits and moisture surpluses. Comparison of modeled Albian and modern P-E curves suggest amplification of the Albian moisture deficit between 7.5 and 30°N latitude (up to 65% greater), and amplified Albian moisture surplus in the mid to high latitudes (up to 45% greater). The tropical moisture deficit is calculated to represent an average heat loss of approximately 74 W/m² at 10°N paleolatitude (present 16.5 W/m²), at 45°N an average heat gain of 83 W/m² (present 23 W/m²); and at 75°N an Albian heat gain of 19 W/m² (present 4 W/m²). These quantitative estimates of increased poleward heat transfer by H₂O vapor (latent heat flux-LHF) during the mid-Cretaceous greenhouse warming may help to explain the reduced equator-to-pole temperature gradients. The hydrologic cycle was mass-balance modeled from the empirical paleolatitudinal trend (34°-75°N) in mid-Cretaceous meteoric sphaerosiderite d¹⁸O values of the Western Interior Basin, which is steeper and lighter than the modern theoretical gradient. The model was calibrated using modern d¹⁸O values, and the results suggest that mid-Cretaceous precipitation rates exceeded modern mid-high latitude rates (156-220% greater in mid latitudes [2600-3300 mm/yr], and 99% greater at high latitudes [550 mm/yr]). The modeling results and LHF estimates suggest enhanced aridity near the Hadley Cell Boundary (25°-30°N). Paleosol carbonates from the Antler's Formation of Oklahoma and the Glenrose Formation of Texas show positive linear covariant trends (PLCT) in d¹⁸O vs. d¹³C values that resulted from evaporative enrichment of vadose pore-waters duing calcite precipitation. Meteoric calcite lines defined for these paleosols extend the latitudinal transect in mid-Cretaceous meteoric d¹⁸O values to 25ºN paleolatitude, and the PLCT's will aid in quantifying evaporation rates and refining mid-Cretaceous LHF estimates.